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"A reasonable assumption can be made that the two of them were members of the militant wing of Hezbollah," he said.

The six-month-long Bulgarian investigation, which worked together with Israeli and US intelligence experts, found that the suspects carried Canadian and Australian passports and had lived in Lebanon between 2006 and 2010.

Other evidence also suggests that Hezbollah put up the money for the operation, which killed five Israeli tourists and a Bulgarian bus driver, Tsvetanov said.

For her part, EU foreign policy chief Catherine Ashton said also on Tuesday that the EU will need to review the evidence before taking action.

"The implications of the investigation need to be assessed seriously as they relate to a terrorist attack on EU soil, which resulted in the killing and injury of innocent civilians," Ashton said in a statement, which refrained from naming the Lebanese group.

The Bulgarian announcement prompted fresh calls from Israel and the US for the EU to list Hezbollah as a terrorist entity.

The move would forbid EU diplomats to meet with its members and make it illegal for sympathisers in Europe to send it money.

Israel's Prime Minister Benjamin Netanyahu said Hezbollah is complicit in a worldwide campaign of terror orchestrated by its ally and paymaster, Iran, which has now encroached on EU territory.

"There is only one Hezbollah. It is one organisation with one leadership," he said.

US secretary of state John Kerry said the Bulgarian finding "is clear and unequivocal" and that the suicide attack bears striking similarities to other plots in Asia last year.

"We strongly urge other governments around the world - and particularly our partners in Europe - to take immediate action to crack down on Hezbollah," he noted.

An EU Council cell specialising in counter-terrorism - the CP931 group, named after an EU "common position" from 2001 - is likely to hold an extraordinary meeting with Bulgarian experts in the next few days or weeks.

But any decision to list Hezbollah would be taken by EU foreign ministers, following political discussions on its implications for EU foreign policy by EU diplomats in the so-called Coreper, MaMa and Relex Council units.

Some member states, such as the UK, already make a legal distinction between the political and military branches of Hezbollah.

Others, such as France, the former colonial power in Lebanon, are reluctant to list it in case the move further destabilises the fragmented and war-scarred country, as well as the wider region.

"There is no automatic listing just because you have been behind a terrorist attack. It's not only the legal requirement that you have to take into consideration, it's also a political assessment of the context and the timing," EU counter-terrorism co-ordinator, Gilles de Kerchove, told EUobserver in a recent interview.